PCAA Meteorology cheat sheet

Composition by volume of dry air: nitrogen 78.09%, oxygen 20.95%, argon 0.93%, carbon dioxide 0.04%; water vapour is variable, 0-4% near the surface.

Standard atmosphere (ISA): MSL pressure 1013.25 hPa, temperature 15 °C, density 1.225 kg/m³, lapse rate 1.98 °C/1000 ft (6.5 °C/km) up to 11 km (36,090 ft).

ISA tropopause: 11 km, temperature -56.5 °C; isothermal layer from 11 to 20 km; temperature rises again in the stratosphere due to ozone absorption of UV.

Troposphere holds about 75% of atmospheric mass and nearly all water vapour and weather; it is the layer of decreasing temperature and strong vertical mixing.

Pressure halves roughly every 5500 m (18,000 ft) in the lower atmosphere: 500 hPa near 18,000 ft, 300 hPa near 30,000 ft, 200 hPa near 39,000 ft.

Temperature decreases with height in the troposphere, is constant in the lower stratosphere, and increases above about 20 km; the tropopause is higher and colder over the equator than over the poles.

Density altitude = pressure altitude + 120 ft per °C that the actual temperature exceeds ISA temperature at that level.

Ozone layer concentrated 15-35 km absorbs ultraviolet radiation; its absence at the surface would make life impossible and its presence warms the stratosphere.

Dry adiabatic lapse rate (DALR) = 3 °C/1000 ft (9.8 °C/km); saturated adiabatic lapse rate (SALR) varies 1.5-3 °C/1000 ft, smallest near 2 °C/1000 ft in warm moist air.

Environmental lapse rate (ELR) is the actual measured profile; compare with DALR and SALR to classify stability.

Absolutely stable: ELR < SALR (typically < 1.5 °C/1000 ft); absolutely unstable: ELR > DALR (> 3 °C/1000 ft); conditionally unstable: SALR < ELR < DALR.

Lifting condensation level (LCL) ≈ 400 ft per °C of surface dewpoint spread (temperature minus dewpoint); dewpoint spread of 5 °C gives cloud base near 2000 ft.

Relative humidity = (actual vapour pressure / saturation vapour pressure) × 100; RH rises as temperature falls at constant moisture content.

Convective condensation level (CCL) is the height where a surface parcel becomes saturated by daytime heating; cumulus bases form there.

Level of free convection (LFC) is where a lifted parcel becomes warmer than the environment; above it the parcel accelerates upward freely until the equilibrium level (EL).

Latent heat: evaporation absorbs about 2.5 MJ/kg, condensation releases it; this fuels thunderstorms and warms saturated rising air.

QNH is the altimeter setting that makes the altimeter read elevation above MSL at the aerodrome; QFE makes it read height above the aerodrome reference point.

QNE is the standard setting 1013.25 hPa (29.92 inHg); the altitude indicated on QNE is flight level (FL), e.g. FL350.

Transition altitude is where climb changes from QNH to standard; transition level is where descent changes from standard to QNH; the layer between is the transition layer.

Altimeter error: flying from high pressure to low pressure without resetting the subscale makes the aircraft lower than indicated; from low to high, higher than indicated.

True altitude is lower than indicated when the aircraft is colder than ISA; roughly 4% error per 10 °C of ISA deviation, i.e. about 120 ft per 1000 ft at 10 °C cold.

Pressure altitude = indicated altitude when subscale set to 1013.25 hPa; density altitude = pressure altitude corrected for non-standard temperature.

High density altitude (hot, high, humid) reduces engine power, propeller/thrust and lift, increasing take-off and landing distances.

Station pressure is the actual pressure at the station; QNH is derived from station pressure using the station elevation and ISA temperature profile.

Pressure gradient force (PGF) acts from high to low pressure perpendicular to isobars; its strength is proportional to isobar spacing (tight spacing = strong wind).

Coriolis force is perpendicular to wind, to the right in the Northern Hemisphere, proportional to wind speed and sin(latitude); zero at the equator.

Geostrophic wind is the balance of PGF and Coriolis above the friction layer (above about 2000 ft); it blows parallel to straight isobars.

In the friction layer, surface wind crosses isobars toward low pressure at about 30-45° over land and 10-20° over sea; wind backs with height in the Northern Hemisphere.

Buys Ballot's law: in the Northern Hemisphere, with your back to the wind, low pressure is on your left.

Sea breeze blows from sea to land by day; land breeze from land to sea at night; both are driven by differential heating and are strongest in the afternoon.

Anabatic (upslope) winds blow up valley sides by day; katabatic (downslope) winds flow down at night, producing cold pooling in valleys.

Jet streams: polar front jet near 30,000-40,000 ft, subtropical jet near 40,000 ft; wind shear and CAT are most frequent on the cold (polar) side of the jet core.

Cloud classification by height: high (cirrus, cirrostratus, cirrocumulus, above 20,000 ft), middle (altostratus, altocumulus, 6500-20,000 ft), low (stratus, stratocumulus, 0-6500 ft); cumulus and cumulonimbus span all levels.

Cumuliform clouds indicate instability (strong vertical development); stratiform clouds indicate stability (layered, weak vertical motion).

Nimbostratus gives continuous steady precipitation; cumulonimbus gives showery precipitation, hail, lightning and gust fronts.

Radiation fog forms by nocturnal cooling under clear skies and light wind, usually in autumn/winter, and dissipates after sunrise; it needs a dewpoint spread of about 2 °C or less.

Advection fog forms when warm moist air moves over a colder surface; it can persist day and night and needs wind of about 5-15 kt.

Upslope fog forms when moist air is forced up rising terrain; it is common on the windward slopes of hills and mountains.

Steam fog (Arctic sea smoke) forms when cold air passes over warm water; mixing of cold air and warm moist air saturates it.

Frontal fog forms when rain evaporates into cooler air below cloud and saturates it; it is common ahead of warm fronts.

Air mass classification: cA (continental Arctic), cP (continental Polar), mP (maritime Polar), cT (continental Tropical), mT (maritime Tropical); source region determines temperature and humidity.

Cold front: steep slope (about 1:50), fast movement, narrow band of heavy showery rain, gusty wind, veering wind, pressure rising after passage, visibility improving.

Warm front: shallow slope (about 1:150), slow movement, wide band of steady rain, veering wind, pressure falling then steady, visibility poor in precipitation.

Occluded front: cold front catches up with warm front; warm sector lifted off the ground; mixed weather with rain and often poor visibility.

Stationary front: little movement, prolonged cloud and rain over the same area, often with low cloud and fog.

Trough: elongated area of low pressure; associated with showers, squally wind and often a wind shift; marked on charts as a dashed line.

Ridge: elongated area of high pressure; associated with subsidence, clear skies and good visibility, though often with low-level inversions.

Squall line: line of active thunderstorms ahead of or along a cold front, producing severe gusts, hail and heavy rain.

Structural icing requires visible moisture and airframe temperature between 0 °C and -20 °C; most severe in the -2 to -10 °C range.

Clear ice forms from large supercooled droplets in cumuliform cloud, often at 0 to -10 °C; rime ice forms from small droplets in stratiform cloud at -10 to -20 °C.

Freezing rain and freezing drizzle produce the most rapid and severe airframe icing; avoid them entirely.

Icing effects: lift loss, weight increase, drag increase, propeller/engine intake icing, pitot and static port blockage, and control surface restriction.

CAT (clear air turbulence) occurs in jet streams, especially on the cold side of the core, near tropopause folds, and over mountain waves.

Low-level wind shear: change of wind speed/direction over short distance; most critical below 2000 ft AGL on approach and departure, often with thunderstorms and frontal passages.

Microburst: downdraft less than 4 km across, life cycle 5-15 min, can produce a sudden 30-90 kt loss of headwind and is lethal on approach.

Thunderstorm hazards: hail, lightning, severe turbulence, icing, microbursts, gust fronts, tornadoes; avoid by at least 20 nm (5 nm per 1000 ft of cloud top above 20,000 ft).

METAR issue times: usually hourly at :00 or :30; validity covers the observation period; SPECI issued when significant change occurs (visibility, ceiling, wind, thunderstorm, etc.).

METAR format: wind dddffKT (gusts G), visibility in metres or statute miles, RVR, present weather, cloud amount and height, temperature/dewpoint, QNH, trend.

Cloud amount codes: FEW (1-2 oktas), SCT (3-4), BKN (5-7), OVC (8); BKN or OVC at the lowest layer defines ceiling.

Weather codes: RA rain, SN snow, DZ drizzle, SH showers, TS thunderstorm, GR hail, FZ freezing, BR mist, FG fog, HZ haze, SQ squall, FC funnel cloud.

TAF: validity usually 24-30 hours, issued 4 times daily (00, 06, 12, 18 UTC); BECMG = gradual change, TEMPO = temporary fluctuation, FM = rapid change at a specific time.

SIGMET: issued for severe turbulence, severe icing, severe mountain waves, volcanic ash, tropical cyclones, and other significant hazards; valid for 4 hours (6 hours for volcanic ash).

AIRMET (where used) covers moderate hazards and widespread conditions; PIREP (UA/UUA) reports actual aircraft-observed conditions including turbulence and icing intensity.

Wind shear and microburst alerts (WS, MW) are reported in METAR when detected by low-level wind shear alert systems.

Surface synoptic chart (ASXX): isobars at 4 hPa intervals; fronts, pressure centres, troughs and ridges; symbols for precipitation and present weather.

Upper air charts: 850 hPa (~5000 ft), 700 hPa (~10,000 ft), 500 hPa (~18,000 ft), 300 hPa (~30,000 ft), 250 hPa (~34,000 ft); show height contours, temperature, wind and humidity.

Constant pressure chart interpretation: closely spaced height contours indicate strong wind; wind blows nearly parallel to contours, low heights to the left in the Northern Hemisphere.

Thickness chart (1000-500 hPa): thickness proportional to mean temperature; low thickness indicates cold air and lower snow level.

Significant weather chart (SIGWX): shows fronts, jet streams, CAT areas, icing, CBs, tropopause height and volcanic ash; used for flight planning.

Wind and temperature aloft forecast (FD): gives wind direction/speed and temperature at specified levels and times; used for fuel and route planning.

Prognostic charts (24/48 h forecasts) show expected pressure, fronts and precipitation; compare with actual analysis to judge forecast reliability.

Volcanic ash advisory (VAA) and tropical cyclone advisory (TCA) are issued as special chart products; ash is a total avoidance hazard.

Pakistan has four seasons: winter (Dec-Mar), pre-monsoon/hot season (Apr-Jun), monsoon (Jul-Sep), post-monsoon (Oct-Nov).

Monsoon (Jul-Sep) brings moist southwesterly flow from the Arabian Sea and Bay of Bengal; heaviest rain on the windward slopes of the Himalayas and Karakoram, causing floods in Punjab and Sindh.

Western disturbances (WDs) are low-pressure systems from the Mediterranean/Caspian that bring winter rain and snow to northern Pakistan and the western borderlands.

Pre-monsoon (May-June) heat lows over Balochistan and Sindh produce dust storms, dust-raising winds and extreme heat; Jacobabad and Sibi often exceed 50 °C.

Karachi coastal climate is moderated by sea breeze; fog and low stratus occur in winter mornings, and monsoon rain peaks in July-August.

Northern areas (Gilgit-Baltistan, Khyber Pakhtunkhwa) experience mountain-induced turbulence, valley winds, and rapid weather changes; snow and icing are common above 10,000 ft in winter.

Dust and sandstorms (haboob-type) reduce visibility to near zero in Balochistan and Cholistan, especially in summer afternoons.

Tropical cyclones in the Arabian Sea most often affect the Sindh and Balochistan coast in May-June and October-November, bringing heavy rain, storm surge and gale winds.

Composition by volume of dry air: nitrogen 78.09%, oxygen 20.95%, argon 0.93%, carbon dioxide 0.04%; water vapour is variable, 0-4% near the surface.

Standard atmosphere (ISA): MSL pressure 1013.25 hPa, temperature 15 °C, density 1.225 kg/m³, lapse rate 1.98 °C/1000 ft (6.5 °C/km) up to 11 km (36,090 ft).

ISA tropopause: 11 km, temperature -56.5 °C; isothermal layer from 11 to 20 km; temperature rises again in the stratosphere due to ozone absorption of UV.

Troposphere holds about 75% of atmospheric mass and nearly all water vapour and weather; it is the layer of decreasing temperature and strong vertical mixing.

Pressure halves roughly every 5500 m (18,000 ft) in the lower atmosphere: 500 hPa near 18,000 ft, 300 hPa near 30,000 ft, 200 hPa near 39,000 ft.

Temperature decreases with height in the troposphere, is constant in the lower stratosphere, and increases above about 20 km; the tropopause is higher and colder over the equator than over the poles.

Density altitude = pressure altitude + 120 ft per °C that the actual temperature exceeds ISA temperature at that level.

Ozone layer concentrated 15-35 km absorbs ultraviolet radiation; its absence at the surface would make life impossible and its presence warms the stratosphere.

Dry adiabatic lapse rate (DALR) = 3 °C/1000 ft (9.8 °C/km); saturated adiabatic lapse rate (SALR) varies 1.5-3 °C/1000 ft, smallest near 2 °C/1000 ft in warm moist air.

Environmental lapse rate (ELR) is the actual measured profile; compare with DALR and SALR to classify stability.

Absolutely stable: ELR < SALR (typically < 1.5 °C/1000 ft); absolutely unstable: ELR > DALR (> 3 °C/1000 ft); conditionally unstable: SALR < ELR < DALR.

Lifting condensation level (LCL) ≈ 400 ft per °C of surface dewpoint spread (temperature minus dewpoint); dewpoint spread of 5 °C gives cloud base near 2000 ft.

Relative humidity = (actual vapour pressure / saturation vapour pressure) × 100; RH rises as temperature falls at constant moisture content.

Convective condensation level (CCL) is the height where a surface parcel becomes saturated by daytime heating; cumulus bases form there.

Level of free convection (LFC) is where a lifted parcel becomes warmer than the environment; above it the parcel accelerates upward freely until the equilibrium level (EL).

Latent heat: evaporation absorbs about 2.5 MJ/kg, condensation releases it; this fuels thunderstorms and warms saturated rising air.

QNH is the altimeter setting that makes the altimeter read elevation above MSL at the aerodrome; QFE makes it read height above the aerodrome reference point.

QNE is the standard setting 1013.25 hPa (29.92 inHg); the altitude indicated on QNE is flight level (FL), e.g. FL350.

Transition altitude is where climb changes from QNH to standard; transition level is where descent changes from standard to QNH; the layer between is the transition layer.

Altimeter error: flying from high pressure to low pressure without resetting the subscale makes the aircraft lower than indicated; from low to high, higher than indicated.

True altitude is lower than indicated when the aircraft is colder than ISA; roughly 4% error per 10 °C of ISA deviation, i.e. about 120 ft per 1000 ft at 10 °C cold.

Pressure altitude = indicated altitude when subscale set to 1013.25 hPa; density altitude = pressure altitude corrected for non-standard temperature.

High density altitude (hot, high, humid) reduces engine power, propeller/thrust and lift, increasing take-off and landing distances.

Station pressure is the actual pressure at the station; QNH is derived from station pressure using the station elevation and ISA temperature profile.

Pressure gradient force (PGF) acts from high to low pressure perpendicular to isobars; its strength is proportional to isobar spacing (tight spacing = strong wind).

Coriolis force is perpendicular to wind, to the right in the Northern Hemisphere, proportional to wind speed and sin(latitude); zero at the equator.

Geostrophic wind is the balance of PGF and Coriolis above the friction layer (above about 2000 ft); it blows parallel to straight isobars.

In the friction layer, surface wind crosses isobars toward low pressure at about 30-45° over land and 10-20° over sea; wind backs with height in the Northern Hemisphere.

Buys Ballot's law: in the Northern Hemisphere, with your back to the wind, low pressure is on your left.

Sea breeze blows from sea to land by day; land breeze from land to sea at night; both are driven by differential heating and are strongest in the afternoon.

Anabatic (upslope) winds blow up valley sides by day; katabatic (downslope) winds flow down at night, producing cold pooling in valleys.

Jet streams: polar front jet near 30,000-40,000 ft, subtropical jet near 40,000 ft; wind shear and CAT are most frequent on the cold (polar) side of the jet core.

Cloud classification by height: high (cirrus, cirrostratus, cirrocumulus, above 20,000 ft), middle (altostratus, altocumulus, 6500-20,000 ft), low (stratus, stratocumulus, 0-6500 ft); cumulus and cumulonimbus span all levels.

Cumuliform clouds indicate instability (strong vertical development); stratiform clouds indicate stability (layered, weak vertical motion).

Nimbostratus gives continuous steady precipitation; cumulonimbus gives showery precipitation, hail, lightning and gust fronts.

Radiation fog forms by nocturnal cooling under clear skies and light wind, usually in autumn/winter, and dissipates after sunrise; it needs a dewpoint spread of about 2 °C or less.

Advection fog forms when warm moist air moves over a colder surface; it can persist day and night and needs wind of about 5-15 kt.

Upslope fog forms when moist air is forced up rising terrain; it is common on the windward slopes of hills and mountains.

Steam fog (Arctic sea smoke) forms when cold air passes over warm water; mixing of cold air and warm moist air saturates it.

Frontal fog forms when rain evaporates into cooler air below cloud and saturates it; it is common ahead of warm fronts.

Air mass classification: cA (continental Arctic), cP (continental Polar), mP (maritime Polar), cT (continental Tropical), mT (maritime Tropical); source region determines temperature and humidity.

Cold front: steep slope (about 1:50), fast movement, narrow band of heavy showery rain, gusty wind, veering wind, pressure rising after passage, visibility improving.

Warm front: shallow slope (about 1:150), slow movement, wide band of steady rain, veering wind, pressure falling then steady, visibility poor in precipitation.

Occluded front: cold front catches up with warm front; warm sector lifted off the ground; mixed weather with rain and often poor visibility.

Stationary front: little movement, prolonged cloud and rain over the same area, often with low cloud and fog.

Trough: elongated area of low pressure; associated with showers, squally wind and often a wind shift; marked on charts as a dashed line.

Ridge: elongated area of high pressure; associated with subsidence, clear skies and good visibility, though often with low-level inversions.

Squall line: line of active thunderstorms ahead of or along a cold front, producing severe gusts, hail and heavy rain.

Structural icing requires visible moisture and airframe temperature between 0 °C and -20 °C; most severe in the -2 to -10 °C range.

Clear ice forms from large supercooled droplets in cumuliform cloud, often at 0 to -10 °C; rime ice forms from small droplets in stratiform cloud at -10 to -20 °C.

Freezing rain and freezing drizzle produce the most rapid and severe airframe icing; avoid them entirely.

Icing effects: lift loss, weight increase, drag increase, propeller/engine intake icing, pitot and static port blockage, and control surface restriction.

CAT (clear air turbulence) occurs in jet streams, especially on the cold side of the core, near tropopause folds, and over mountain waves.

Low-level wind shear: change of wind speed/direction over short distance; most critical below 2000 ft AGL on approach and departure, often with thunderstorms and frontal passages.

Microburst: downdraft less than 4 km across, life cycle 5-15 min, can produce a sudden 30-90 kt loss of headwind and is lethal on approach.

Thunderstorm hazards: hail, lightning, severe turbulence, icing, microbursts, gust fronts, tornadoes; avoid by at least 20 nm (5 nm per 1000 ft of cloud top above 20,000 ft).

METAR issue times: usually hourly at :00 or :30; validity covers the observation period; SPECI issued when significant change occurs (visibility, ceiling, wind, thunderstorm, etc.).

METAR format: wind dddffKT (gusts G), visibility in metres or statute miles, RVR, present weather, cloud amount and height, temperature/dewpoint, QNH, trend.

Cloud amount codes: FEW (1-2 oktas), SCT (3-4), BKN (5-7), OVC (8); BKN or OVC at the lowest layer defines ceiling.

Weather codes: RA rain, SN snow, DZ drizzle, SH showers, TS thunderstorm, GR hail, FZ freezing, BR mist, FG fog, HZ haze, SQ squall, FC funnel cloud.

TAF: validity usually 24-30 hours, issued 4 times daily (00, 06, 12, 18 UTC); BECMG = gradual change, TEMPO = temporary fluctuation, FM = rapid change at a specific time.

SIGMET: issued for severe turbulence, severe icing, severe mountain waves, volcanic ash, tropical cyclones, and other significant hazards; valid for 4 hours (6 hours for volcanic ash).

AIRMET (where used) covers moderate hazards and widespread conditions; PIREP (UA/UUA) reports actual aircraft-observed conditions including turbulence and icing intensity.

Wind shear and microburst alerts (WS, MW) are reported in METAR when detected by low-level wind shear alert systems.

Surface synoptic chart (ASXX): isobars at 4 hPa intervals; fronts, pressure centres, troughs and ridges; symbols for precipitation and present weather.

Upper air charts: 850 hPa (~5000 ft), 700 hPa (~10,000 ft), 500 hPa (~18,000 ft), 300 hPa (~30,000 ft), 250 hPa (~34,000 ft); show height contours, temperature, wind and humidity.

Constant pressure chart interpretation: closely spaced height contours indicate strong wind; wind blows nearly parallel to contours, low heights to the left in the Northern Hemisphere.

Thickness chart (1000-500 hPa): thickness proportional to mean temperature; low thickness indicates cold air and lower snow level.

Significant weather chart (SIGWX): shows fronts, jet streams, CAT areas, icing, CBs, tropopause height and volcanic ash; used for flight planning.

Wind and temperature aloft forecast (FD): gives wind direction/speed and temperature at specified levels and times; used for fuel and route planning.

Prognostic charts (24/48 h forecasts) show expected pressure, fronts and precipitation; compare with actual analysis to judge forecast reliability.

Volcanic ash advisory (VAA) and tropical cyclone advisory (TCA) are issued as special chart products; ash is a total avoidance hazard.

Pakistan has four seasons: winter (Dec-Mar), pre-monsoon/hot season (Apr-Jun), monsoon (Jul-Sep), post-monsoon (Oct-Nov).

Monsoon (Jul-Sep) brings moist southwesterly flow from the Arabian Sea and Bay of Bengal; heaviest rain on the windward slopes of the Himalayas and Karakoram, causing floods in Punjab and Sindh.

Western disturbances (WDs) are low-pressure systems from the Mediterranean/Caspian that bring winter rain and snow to northern Pakistan and the western borderlands.

Pre-monsoon (May-June) heat lows over Balochistan and Sindh produce dust storms, dust-raising winds and extreme heat; Jacobabad and Sibi often exceed 50 °C.

Karachi coastal climate is moderated by sea breeze; fog and low stratus occur in winter mornings, and monsoon rain peaks in July-August.

Northern areas (Gilgit-Baltistan, Khyber Pakhtunkhwa) experience mountain-induced turbulence, valley winds, and rapid weather changes; snow and icing are common above 10,000 ft in winter.

Dust and sandstorms (haboob-type) reduce visibility to near zero in Balochistan and Cholistan, especially in summer afternoons.

Tropical cyclones in the Arabian Sea most often affect the Sindh and Balochistan coast in May-June and October-November, bringing heavy rain, storm surge and gale winds.

Composition by volume of dry air: nitrogen 78.09%, oxygen 20.95%, argon 0.93%, carbon dioxide 0.04%; water vapour is variable, 0-4% near the surface.

Standard atmosphere (ISA): MSL pressure 1013.25 hPa, temperature 15 °C, density 1.225 kg/m³, lapse rate 1.98 °C/1000 ft (6.5 °C/km) up to 11 km (36,090 ft).

ISA tropopause: 11 km, temperature -56.5 °C; isothermal layer from 11 to 20 km; temperature rises again in the stratosphere due to ozone absorption of UV.

Troposphere holds about 75% of atmospheric mass and nearly all water vapour and weather; it is the layer of decreasing temperature and strong vertical mixing.

Pressure halves roughly every 5500 m (18,000 ft) in the lower atmosphere: 500 hPa near 18,000 ft, 300 hPa near 30,000 ft, 200 hPa near 39,000 ft.

Temperature decreases with height in the troposphere, is constant in the lower stratosphere, and increases above about 20 km; the tropopause is higher and colder over the equator than over the poles.

Density altitude = pressure altitude + 120 ft per °C that the actual temperature exceeds ISA temperature at that level.

Ozone layer concentrated 15-35 km absorbs ultraviolet radiation; its absence at the surface would make life impossible and its presence warms the stratosphere.

Dry adiabatic lapse rate (DALR) = 3 °C/1000 ft (9.8 °C/km); saturated adiabatic lapse rate (SALR) varies 1.5-3 °C/1000 ft, smallest near 2 °C/1000 ft in warm moist air.

Environmental lapse rate (ELR) is the actual measured profile; compare with DALR and SALR to classify stability.

Absolutely stable: ELR < SALR (typically < 1.5 °C/1000 ft); absolutely unstable: ELR > DALR (> 3 °C/1000 ft); conditionally unstable: SALR < ELR < DALR.

Lifting condensation level (LCL) ≈ 400 ft per °C of surface dewpoint spread (temperature minus dewpoint); dewpoint spread of 5 °C gives cloud base near 2000 ft.

Relative humidity = (actual vapour pressure / saturation vapour pressure) × 100; RH rises as temperature falls at constant moisture content.

Convective condensation level (CCL) is the height where a surface parcel becomes saturated by daytime heating; cumulus bases form there.

Level of free convection (LFC) is where a lifted parcel becomes warmer than the environment; above it the parcel accelerates upward freely until the equilibrium level (EL).

Latent heat: evaporation absorbs about 2.5 MJ/kg, condensation releases it; this fuels thunderstorms and warms saturated rising air.

QNH is the altimeter setting that makes the altimeter read elevation above MSL at the aerodrome; QFE makes it read height above the aerodrome reference point.

QNE is the standard setting 1013.25 hPa (29.92 inHg); the altitude indicated on QNE is flight level (FL), e.g. FL350.

Transition altitude is where climb changes from QNH to standard; transition level is where descent changes from standard to QNH; the layer between is the transition layer.

Altimeter error: flying from high pressure to low pressure without resetting the subscale makes the aircraft lower than indicated; from low to high, higher than indicated.

True altitude is lower than indicated when the aircraft is colder than ISA; roughly 4% error per 10 °C of ISA deviation, i.e. about 120 ft per 1000 ft at 10 °C cold.

Pressure altitude = indicated altitude when subscale set to 1013.25 hPa; density altitude = pressure altitude corrected for non-standard temperature.

High density altitude (hot, high, humid) reduces engine power, propeller/thrust and lift, increasing take-off and landing distances.

Station pressure is the actual pressure at the station; QNH is derived from station pressure using the station elevation and ISA temperature profile.

Pressure gradient force (PGF) acts from high to low pressure perpendicular to isobars; its strength is proportional to isobar spacing (tight spacing = strong wind).

Coriolis force is perpendicular to wind, to the right in the Northern Hemisphere, proportional to wind speed and sin(latitude); zero at the equator.

Geostrophic wind is the balance of PGF and Coriolis above the friction layer (above about 2000 ft); it blows parallel to straight isobars.

In the friction layer, surface wind crosses isobars toward low pressure at about 30-45° over land and 10-20° over sea; wind backs with height in the Northern Hemisphere.

Buys Ballot's law: in the Northern Hemisphere, with your back to the wind, low pressure is on your left.

Sea breeze blows from sea to land by day; land breeze from land to sea at night; both are driven by differential heating and are strongest in the afternoon.

Anabatic (upslope) winds blow up valley sides by day; katabatic (downslope) winds flow down at night, producing cold pooling in valleys.

Jet streams: polar front jet near 30,000-40,000 ft, subtropical jet near 40,000 ft; wind shear and CAT are most frequent on the cold (polar) side of the jet core.

Cloud classification by height: high (cirrus, cirrostratus, cirrocumulus, above 20,000 ft), middle (altostratus, altocumulus, 6500-20,000 ft), low (stratus, stratocumulus, 0-6500 ft); cumulus and cumulonimbus span all levels.

Cumuliform clouds indicate instability (strong vertical development); stratiform clouds indicate stability (layered, weak vertical motion).

Nimbostratus gives continuous steady precipitation; cumulonimbus gives showery precipitation, hail, lightning and gust fronts.

Radiation fog forms by nocturnal cooling under clear skies and light wind, usually in autumn/winter, and dissipates after sunrise; it needs a dewpoint spread of about 2 °C or less.

Advection fog forms when warm moist air moves over a colder surface; it can persist day and night and needs wind of about 5-15 kt.

Upslope fog forms when moist air is forced up rising terrain; it is common on the windward slopes of hills and mountains.

Steam fog (Arctic sea smoke) forms when cold air passes over warm water; mixing of cold air and warm moist air saturates it.

Frontal fog forms when rain evaporates into cooler air below cloud and saturates it; it is common ahead of warm fronts.

Air mass classification: cA (continental Arctic), cP (continental Polar), mP (maritime Polar), cT (continental Tropical), mT (maritime Tropical); source region determines temperature and humidity.

Cold front: steep slope (about 1:50), fast movement, narrow band of heavy showery rain, gusty wind, veering wind, pressure rising after passage, visibility improving.

Warm front: shallow slope (about 1:150), slow movement, wide band of steady rain, veering wind, pressure falling then steady, visibility poor in precipitation.

Occluded front: cold front catches up with warm front; warm sector lifted off the ground; mixed weather with rain and often poor visibility.

Stationary front: little movement, prolonged cloud and rain over the same area, often with low cloud and fog.

Trough: elongated area of low pressure; associated with showers, squally wind and often a wind shift; marked on charts as a dashed line.

Ridge: elongated area of high pressure; associated with subsidence, clear skies and good visibility, though often with low-level inversions.

Squall line: line of active thunderstorms ahead of or along a cold front, producing severe gusts, hail and heavy rain.

Structural icing requires visible moisture and airframe temperature between 0 °C and -20 °C; most severe in the -2 to -10 °C range.

Clear ice forms from large supercooled droplets in cumuliform cloud, often at 0 to -10 °C; rime ice forms from small droplets in stratiform cloud at -10 to -20 °C.

Freezing rain and freezing drizzle produce the most rapid and severe airframe icing; avoid them entirely.

Icing effects: lift loss, weight increase, drag increase, propeller/engine intake icing, pitot and static port blockage, and control surface restriction.

CAT (clear air turbulence) occurs in jet streams, especially on the cold side of the core, near tropopause folds, and over mountain waves.

Low-level wind shear: change of wind speed/direction over short distance; most critical below 2000 ft AGL on approach and departure, often with thunderstorms and frontal passages.

Microburst: downdraft less than 4 km across, life cycle 5-15 min, can produce a sudden 30-90 kt loss of headwind and is lethal on approach.

Thunderstorm hazards: hail, lightning, severe turbulence, icing, microbursts, gust fronts, tornadoes; avoid by at least 20 nm (5 nm per 1000 ft of cloud top above 20,000 ft).

METAR issue times: usually hourly at :00 or :30; validity covers the observation period; SPECI issued when significant change occurs (visibility, ceiling, wind, thunderstorm, etc.).

METAR format: wind dddffKT (gusts G), visibility in metres or statute miles, RVR, present weather, cloud amount and height, temperature/dewpoint, QNH, trend.

Cloud amount codes: FEW (1-2 oktas), SCT (3-4), BKN (5-7), OVC (8); BKN or OVC at the lowest layer defines ceiling.

Weather codes: RA rain, SN snow, DZ drizzle, SH showers, TS thunderstorm, GR hail, FZ freezing, BR mist, FG fog, HZ haze, SQ squall, FC funnel cloud.

TAF: validity usually 24-30 hours, issued 4 times daily (00, 06, 12, 18 UTC); BECMG = gradual change, TEMPO = temporary fluctuation, FM = rapid change at a specific time.

SIGMET: issued for severe turbulence, severe icing, severe mountain waves, volcanic ash, tropical cyclones, and other significant hazards; valid for 4 hours (6 hours for volcanic ash).

AIRMET (where used) covers moderate hazards and widespread conditions; PIREP (UA/UUA) reports actual aircraft-observed conditions including turbulence and icing intensity.

Wind shear and microburst alerts (WS, MW) are reported in METAR when detected by low-level wind shear alert systems.

Surface synoptic chart (ASXX): isobars at 4 hPa intervals; fronts, pressure centres, troughs and ridges; symbols for precipitation and present weather.

Upper air charts: 850 hPa (~5000 ft), 700 hPa (~10,000 ft), 500 hPa (~18,000 ft), 300 hPa (~30,000 ft), 250 hPa (~34,000 ft); show height contours, temperature, wind and humidity.

Constant pressure chart interpretation: closely spaced height contours indicate strong wind; wind blows nearly parallel to contours, low heights to the left in the Northern Hemisphere.

Thickness chart (1000-500 hPa): thickness proportional to mean temperature; low thickness indicates cold air and lower snow level.

Significant weather chart (SIGWX): shows fronts, jet streams, CAT areas, icing, CBs, tropopause height and volcanic ash; used for flight planning.

Wind and temperature aloft forecast (FD): gives wind direction/speed and temperature at specified levels and times; used for fuel and route planning.

Prognostic charts (24/48 h forecasts) show expected pressure, fronts and precipitation; compare with actual analysis to judge forecast reliability.

Volcanic ash advisory (VAA) and tropical cyclone advisory (TCA) are issued as special chart products; ash is a total avoidance hazard.

Pakistan has four seasons: winter (Dec-Mar), pre-monsoon/hot season (Apr-Jun), monsoon (Jul-Sep), post-monsoon (Oct-Nov).

Monsoon (Jul-Sep) brings moist southwesterly flow from the Arabian Sea and Bay of Bengal; heaviest rain on the windward slopes of the Himalayas and Karakoram, causing floods in Punjab and Sindh.

Western disturbances (WDs) are low-pressure systems from the Mediterranean/Caspian that bring winter rain and snow to northern Pakistan and the western borderlands.

Pre-monsoon (May-June) heat lows over Balochistan and Sindh produce dust storms, dust-raising winds and extreme heat; Jacobabad and Sibi often exceed 50 °C.

Karachi coastal climate is moderated by sea breeze; fog and low stratus occur in winter mornings, and monsoon rain peaks in July-August.

Northern areas (Gilgit-Baltistan, Khyber Pakhtunkhwa) experience mountain-induced turbulence, valley winds, and rapid weather changes; snow and icing are common above 10,000 ft in winter.

Dust and sandstorms (haboob-type) reduce visibility to near zero in Balochistan and Cholistan, especially in summer afternoons.

Tropical cyclones in the Arabian Sea most often affect the Sindh and Balochistan coast in May-June and October-November, bringing heavy rain, storm surge and gale winds.